The pendulum swings and swings again

The saga of NGC 1052-DF2 and DF4 continues.

I'm not going to try and keep up with every development on this one any more because there are just too many. In brief, two galaxies were found that apparently had no dark matter at all. Given their apparent distance of about 20 Mpc, their size and velocity dispersion appear to indicate that their stars are moving so slowly that they could be gravitationally bound all by themselves, with no need of the usual extra mass.

This discovery has been controversial. First there were claims that the velocity dispersions had been measured incorrectly. This never looked terribly convincing, and indeed that idea has gone away. Then there were claims that the distance was wrong. If the galaxy is actually at about 13 Mpc, then the velocity measurements actually do require quite a lot of dark matter, as well as explaining other anomalies like the brightness of the galaxy's globular clusters. This has turned into a game of galactic ping-pong, with each team periodically refuting the other's distance claims with new data or analysis methods.

The present paper is the latest in a long series of attempts to decisively knock out the claim for a 13 Mpc distance. They use the "tip of the red giant branch" method to estimate the distance. Basically, red giant stars ought to be reasonably good standard candles (at least statistically if not individually), meaning that you know their true brightness so can get the corresponding distance. Previous claims using this method, they say, were not deep enough to detect stars at 20 Mpc distances, leading to all sorts of complications in the analysis. So they've gone and got Hubble data which should be more than sufficient to sort this out. If nothing else, their main image is certainly very pretty.

They say the distance to DF2 is actually even slightly higher than their initial claim, at 22 Mpc. DF4 they've previously said is at 20 Mpc, and though there are certainly some errors in all this, they say this is sufficient to say that at least one of them isn't part of the NGC 1052 group. That's significant, because it makes a tidal formation scenario all the less likely. In addition, they mention that they have wide-field deep imaging (not presented here) which does not show the previously-reported tidal tails, so those features are likely spurious.  

Finally, they'd previously made a boo-boo in claiming that these objects refute MOND, because MOND has an external field effect that they didn't account for. But now it looks like both DF2 and DF4 are too far away from their companion galaxies for this to be significant after all. Well, we'll see. Expect lots of angry responses from both standard model and MOND enthusiasts on this one.

A Tip of the Red Giant Branch Distance of 22.1 Mpc to the Dark Matter Deficient Galaxy NGC1052-DF2 from 40 Orbits of Hubble Space Telescope Imaging

The inferred distance is DTRGB=22.1 Mpc, consistent with the previous surface brightness fluctuation distances to the bright elliptical galaxy NGC1052. The new HST distance rules out the idea that some of NGC1052-DF2's unusual properties can be explained if it were at 13 Mpc; instead, it implies that the galaxy's globular clusters are even more luminous than had been derived using the previous distance of 20 Mpc. The distance from NGC1052-DF2 to NGC1052-DF4 is well-determined at 2.1 Mpc, significantly larger than the virial diameter of NGC1052. We discuss the implications for formation scenarios of the galaxies and for the external field effect, which has been invoked to explain the intrinsic dynamics of these objects in the context of modified Newtonian dynamics.

The tale of a tiny, farting, blushing dwarf

Who amongst us doesn't have the decency to look embarrassed when publicly expelling gas ? Galaxies, it turns out, are no less prone to turning a certain shade of red when things get... windy.

Today's paper combines no less than three of my favourite topics : ultra diffuse galaxies, ram pressure stripping, and dark galaxies. UDGs are these large(ish) galaxies which have very few stars, with dark galaxies having no stars at all. The connection between the two, if any, is highly uncertain, and it's possible dark galaxies are actually just clouds of stripped gas and not really galaxies themselves. And RPS is a particular kind of stripping mechanism : the process whereby a galaxy moving through hot, thin gas can build up so much pressure that its own gas can be forced out. In doing so it eventually runs out fuel for star formation, so all the hot, short-lived blue stars quickly die off, leaving behind only the small red ones.

Hence, farting galaxies turn red. Change my mind.

This phenomenon is more-or-less well known in ordinary galaxies, but no-one has yet seen it happening in UDGs. The authors here present a pretty convincing case for the detection of such an event. Just like their bigger, brighter cousins, UDGs suffer the same acute embarrassment when they too are squeezed uncomfortably.

The story actually stars with the detection of a neutral hydrogen (HI) cloud back in 2015. Cannon et al. reported that a few features, this one included, had no obvious optical counterpart. High resolution observations with the VLA revealed that the cloud was suspiciously close to, but not coincident with, a faint optical counterpart. On a quick re-read of Cannon 2015 I'm not quite sure why they thought this was all that odd (displaced gas is a rare but not unknown or unexpected occurrence); interesting yes, strange, not so much.

(Incidentally, I don't like the term "almost dark" that has become popular. There are genuinely dark structures, so I prefer to call these faint but definite systems dim galaxies. But that's by-the-by.)

This current paper reveals a nice connection between the optical object, the HI detection, and some ultra violet extended emission and blobs. The optical galaxy they categorise as satisfying the UDG criteria (which were generally not known about in 2015), albeit marginally. The UV emission (from ionised or excited gas from hot young stars) is slightly displaced from the UDG (by a few kpc, nothing much at all really) roughly on a line from the UDG towards the centre of the Virgo cluster. 

This very neatly fits the "fireball" model of RPS. Galaxies move towards the cluster centre, the ram pressure builds up until stripping begins, and then the tail of stripped gas behind the galaxy can cool and condense and form UV-bright stars. And that's reasonably well-known for larger systems, but never seen before for galaxies like this.

While the UV data gives them information on metallicity (chemical composition), showing clearly that most of the blobs are about what you'd expect from galactic material (and not pristine gas as you'd expect if this were a case of accretion rather than gas loss), their estimated ages are less conclusive. Through a series of really quite tedious modelling, they're able to establish that the peak of RPS most likely occurred around 100 Myr ago or so. They can't see a nice clear age gradient that would really seal the deal on the fireball model, but given the short duration of stripping and the small distance travelled, that's not at all surprising.

There are a lot of very interesting implications from all this. First, they say the galaxy is likely to have been originally a UDG to begin with - not something that formed as a result of cluster processes as some other models indicate. No, they're truly independent systems in their own right, a normal part of the galaxy population and not some cluster-specific thing.

Second, there's a possible connection to the truly "dark galaxies"... well, maybe. At the very least this is a nice example of displaced gas, whereas previously I believe the only other such example in Virgo was M49. 

It'd be extremely satisfying if the totally dark HI clouds could be explained by this, but the third point probably makes this unlikely. The displaced gas still isn't enough to account for how much gas the original galaxy ought to have had (somewhat surprisingly, many UDGs have pretty normal gas contents), so there's been a loss rate of around 4 solar masses per year. That's pretty much exactly the same as seen in much more massive systems. And if this applies to the dark clouds, which don't have much gas left, we're detecting them suspiciously close to the very end of their lives.

(I suppose there could be a selection effect, however. Maybe they only appear as truly isolated clouds just before the last bit of gas evaporates. But this still implies a high production rate of such systems, so this would be an awkward solution at best.)

What's odd about that is that UDGs need to have lower than usual star formation rates, otherwise they'd just be normal galaxies. So, presumably, they have more extended, lower-density gas. It's not in the least surprising that gas removal quenches star formation and make them all embarrassed red, but one might expect the evaporation of lower-density gas to occur at a very different rate to what happens for much denser gas (e.g. it should disperse more quickly and/or change phase to cooler or hotter undetectable gas more readily). But apparently it doesn't. So, is the physics of gas dissolution different, or is there some completely different quenching mechanism at work in UDGs ?

Right now, we've no idea. Interesting as it may be, it'll take a lot more than one farting dwarf to solve all the mysteries of galaxy evolution.

Formation of a red ultra-diffuse galaxy and an almost dark galaxy during a ram-pressure stripping event

We detected a few star-forming blobs in the VESTIGE survey, located at ∼5 kpc from a UDG, namely NGVS 3543, in association with an HI gas cloud AGC 226178, suggesting a recent interaction between this low-surface-brightness system and the surrounding cluster environment. We use a complete set of multi-frequency data including deep optical, UV, and narrow-band Hα imaging and HI data to understand the formation process that gave birth to this peculiar system.